Mode-locked frequency doubled laser

ABSTRACT

This laser comprises an intracavity nonlinear crystal that converts fundamental frequency laser light to second harmonic light and is also modulated to mode lock the laser. The modulation voltage, and thus the modulation depth, is varied to optimize the magnitude of the second harmonic power produced by the crystal.

Muted States Fatent Foster et al.

[54] MODE-LOCKED FREQUENCY DOUBLED LASER [72] Inventors: Jack D. Foster,Los Altos; Larry M.

Osterluk, Mountain View, both of Calif.

[73] Assignee: Sylvauia Electric Products Inc.

[22] Filed: July 2, 1969 [2l] Appl. No.: 838,440

[52] U.S. Cl. ..331/94.5, 330/43 [5]] ..H0ls 3/10 [58] Field ofSear'ch..33l/94.5; 330/43 [56] References Cited UNITED STATES PATENTS 3,412,251"968 Hargrove ..33l/94.5 X

OTHER PUBLICATIONS Wright, Enhancement of Second Harmonic... LaserCavity,

' OPTICAL PUMP souncs 5] Mar. 7, 1972 Proc. IEEE,V01. 5l,No. ll,Nov.l963,p. 1663.

Bass et al., Reproducible...Mode Locked Laser," IEEE J. Quantum Electr.Vol. QE-3 No. 11, Nov. 1967, pp. 621-- 6. Kohn et al., SecondI-Iarmonic...Laser," Applied Phys. Lett., Vol. 8, No.9, 1 May 1966, pp.231- 233.

Sofier et al., Modulation of...Condition," Physics Letters, Vol. 24A,No.5, 27 Feb. 1967, PP. 282- 3.

Primary Examiner-Ronald L. Wibert Assistant Examiner-R. J. WebsterAttorney-Norman .l. OMalley, John F. Lawler and Russell A. Cannon [57]ABSTRACT This laser comprises an intracavity nonlinear crystal thatconverts fundamental frequency laser light to second harmonic light andis also modulated to mode lock the laser. The modulation voltage, andthus the modulation depth, is varied to optimize the magnitude of thesecond harmonic'power produced by the crystal.

SECOND HARMONIC SECOND HARMONIC Patented March 7, 1972 I 3,648,193

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MODULATION DEPTH a: F I Q 4 m OPTIMUM g INVENTORS JACK D. FOSTER LARRYM. OSTERINK MODULATION DEPTH AGENT MODE-LOCKED FREQUENCY DOUBLED LASERBACKGROUND OF THE INVENTION This invention relates to lasers and moreparticularly to a mode-locked frequency doubled laser.

When light enters a crystal, it induces a dipole in the crystal thatabsorbs and then reradiates an incident photon. This action causes adecrease in the transit time of the light through the crystal andaccounts for the indices of refraction and changes in the polarizationof the light by birefringent crystals. In nonlinear crystals such asbarium sodium niobate (Ba NaNb O and lithium niobate (LiNbo however, theinduced polarization in the crystal has a component proportional to thesquare of the incident optical field. It is therefore possible to obtainsecond harmonic generation or frequency doubling with these nonlinearcrystals since a dipole induced therein can absorb two incident photonsand then reradiate one photon of twice the energy, i.e., twice thefrequency, of the incident signal. Ihis process can be made quiteefficient by phase matching the generated harmonic signal with theincident signal by using the birefringence of the crystal to compensatefor dispersion. In order to increase the probability of an induceddipole generating a second harmonic photon, it is necessary to increasethe probability that the dipole will absorb two incident photons almostsimultaneously. The latter probability may be increased by placing thenonlinear crystal inside the laser cavity where the optical fieldstrengths (i.e., photo densities) are large. Also, this may beaccomplished by increasing the density of incident photons in either thespatial or the time domain.

When second harmonic light is generated in the laser cavity, thefrequency-doubled light operates as a loss to the cavity. Stateddifferently, the fundamental laser photons converted into secondharmonic photons are no longer available to stimulate emission in thelasing medium. If the number of fundamental frequency photons that areconverted to second harmonic photons is too great, the intracavity laserloss will be greater than the gain and lasing will be extinguished.Thus, there is an optimum amount of coupling of fundamental photons intosecond harmonic photons. In general, the optimum percentage conversionof fundamental laser light into second harmonic light is equal to theoptimum mirror transmission loss of the laser.

A prior art technique for optimizing the coupling of fundamental photonsto second harmonic photons is to change the laser beam diameter byvarying the curvature and/or axial spacing of the laser cavity mirrors.These variations cause a change in the power density of the fundamentallaser beam in the nonlinear crystal until maximum second harmonic poweroutput is obtained as indicated, for example, on a power meter. Thistechnique requires precise mechanical adjustments and optical controlwhich is difficult to achieve due to complicated thermal opticalefi'ects in the crystal and laser rod.

SUMMARY OF INVENTION An object of this invention is the provision of asimple method of enhancing generation of second harmonic light in afrequency doubled laser. Another object is the provision of a simple andimproved method of adjusting a frequency-doubled laser for optimizingthe magnitude of second harmonic power generated thereby.

In accordance with this invention, a frequency-doubled laser is modelocked to bunch the laser photons and thus increase the probability ofgenerating second harmonic light. The modulation voltage for modelocking the laser is varied to optimize the generation of secondharmonic light.

DESCRIPTION OF DRAWINGS FIG. 1 is a schematic block diagram of laserembodying this invention;

FIG. 2 is a schematic diagram illustrating the orientation of thenonlinear crystal in the laser cavity of FIG. 1;

FIG. 3 is a graph illustrating the enhancement in generated secondharmonic light as a result of mode locking the laser;

FIG. 4 is a graph illustrating the variation in generated secondharmonic power as a function of modulation depth; and

FIG. 5 is a schematic block diagram of a modified form of thisinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS The mode-locked laser illustratedin FIG. 1 comprises an active laser material 10 located in a high-Qresonant cavity defined by mirrors 11 and 12 which are spaced fromopposite ends of the laser material. Mirror 11 is totally reflecting atthe fundamental laser frequency and the second harmonic thereof. Mirror12 is totally reflecting at the fundamental laser frequency. It istotally transmitting, however, at the second harmonic of the fundamentallaser frequency. The laser material may, by way of example, be acylindrical rod of yttrium aluminum garnet which is doped withneodymium. Light rays 14 from an optical pump source 15 excite theneodymium atoms to produce a population inversion and lasing in the rod10. The pump source may be a tungsten filament or an arc lamp that isoperated continuously.

An optical modulator 17 is also located in the cavity and is axiallyaligned with the laser rod and the mirrors. The modulator comprises arectangular electro-optic crystal 18 having planer parallel electrodes19 and 20 bonded to opposite sides thereof. The electrodes areelectrically connected to terminals of a source 23 of variablemodulation voltage. Source 23 comprises a tunable radiofrequencyoscillator which produces a variable frequency sinusoidal modulationvoltage. The modulation frequency is tuned to the laser inter modefrequency c/2L, where c is the velocity of light and L is the cavitylength, to mode lock the laser.

The electro-optic crystal 18 must simultaneously operate as a frequencydoubler to generate second harmonic light and a phase modulator to modelock the laser. Thus, the crystal must have a nonlinear effect along atleast one axis thereof to produce second harmonic light. The crystalmust also have an electro-optic effect associated with the same axis inorder to produce a phase retardation and mode lock the laser. Suchcrystals are described by J. E. Geusic et al. in the article, TheNonlinear Optical Properties of Ba NaNb o Applied Physics Letters, Vol.II, page 269, (1 Nov. 1967), and Vol. 12, page 224, (15 Mar. 1968). Byway of example, crystal 18 may be a rectangular crystal of barium sodiumniobate or lithium niobate. The specific orientation of a lithiumniobate crystal in the laser cavity is illustrated in FIG. 2. Theextraordinary axis of the crystal is aligned with the applied electricfield (i.e., orthogonal to the planes of the electrodes 19 and 20). Theordinary axis of the crystal is aligned orthogonal to the longitudinalaxis of the laser and orthogonal to the electric field in the crystal. ABrewster polarizer 25 is located in the cavity between the laser mediumand the crystal to produce high loss for extraordinary polarization andlow loss for ordinary polarization. This causes the laser to oscillatewith ordinary fundamental waves which phase match with generatedextraordinary second harmonic waves.

The electric field applied to the crystal has a frequency substantiallyequal to the laser intermode frequency c/2L in order to mode lock thelaser and thereby to enhance the generation of second harmonic photonsand frequency doubling. The number of laser cavity modes that are lockeddepends upon the modulation depth (applied field and coincidence withc/2L). Many locked modes produce tight bunching of laser photons andmaximum second harmonic conversion. Fewer locked modes reduces thesecond harmonic conversion. The modulation voltage on the crystal,therefore, provides control of the second harmonic generation tooptimumly couple power from the laser.

The optimum second harmonic power obtainable with a particular lasermaterial and nonlinear crystal is determined experimentally. The powermay be varied by adjusting the cavity length or the radius of curvatureof the cavity mirrors until it is optimum. Since all crystals and laserrods do not have the same characteristics, it is difficult to buildseveral lasers that are identical. [t is therefore desirable to have ameans of conveniently tuning a packaged laser having a fixed physicalstructure to produce optimum second harmonic power.

in accordance with this invention. the laser cavity is tlesigned so thatthe second harmonic power produced thereby is approximately optimum. Themodulation voltage is then tuned to the laser intermode frequency c/2Lto mode lock the laser. This causes the output of the laser to be atrain of optical pulses which are temporally bunched photons thatenhance the probability of a second harmonic photon being generated.Curves 28 and 29 illustrate the second harmonic power enhancementproduced by mode locking a multitransverse mode laser and a singlespatial mode laser. respectively, as a function of the peak single passphase retardation or modulation depth 8 Curves 28 and 29 reveal that asignificant increase or enhancement in second harmonic power is pbtainedby mode locking the laser. The enhancement is thown relative to a freerunning nonmode-locked laser.

The magnitude of the modulation voltage. and thus the modulation depth,is varied to obtain optimum generation of second harmonic light. Thevariation in the power of the second harmonic light as a function of themodulation depth is represented by curve 30 in FIG. 4. The frequency ofthe modulation voltage could also be varied slightly to accomplish thesame result.

A modified form of this invention lS illustrated in FIG. wherein primedreference characters refer to similar components in the laser in FIG. 1.The lasers illustrated in FIGS. 1 and 5 are similar except that thelatter comprises a nonlinear trystal 31 for producing second harmoniclight and an electropptic crystal 32 for mode locking. The crystals arelocated in the cavity and are axially aligned with the laser rod 10 andthe tavity mirrors 11' and 12.

W hat is claimed is:

ill. The method of doubling the optical frequency of a laser consistingof the steps of:

positioning a single nonlinear crystal in the intracavity path tti" thelaser beam. and suitably modulating said crystal to produce mode lockingand frequency doubling of the .aser beam.

.2. Apparatus for producing a laser beam having a frequency that is thesecond harmonic of the fundamental laser frequency comprising,

i1 laser oscillator comprising a laser medium and spaced mirl'Ol'Sdefining a laser-resonant cavity of predetermined tength determinativeof the fundamental frequency of the laser beam. and

a single electro-optic crystal disposed within said cavity forsimultaneously mode locking said beam and for interacting with thelatter to produce the optical second harmonic thereof. and means forelectrically modulating said crystal.

3. The apparatus according to claim 2 in which one of said mirrors istotally reflecting for a laser beam at both the fundamental frequencyand the second harmonic thereof, the other of said mirrors being totallyreflecting for a laser beam at the fundamental frequency and at leastpartially transmitting at the second harmonic thereof.

J. The apparatus according to claim 2 in which said crystal is abirefringent element, having an ordinary axis and an extraordinary axis.

a polarizer within said cavity aligned with the laser beam,

said polarizer having a polarization plane aligned with one of said axesof said element whereby the fundamental mode of the laser beamoscillates in the plane of said polarizer and the second harmonicthereof oscillates in the plane of the other of said element axes.

5. The apparatus according to claim 2 wherein the means for electricallymodulating comprises a source of modulation voltage having a frequencyapproximately equal to the difference between the frequencies ofadjacent modes in the laser beam.

means for coupling said voltage from the modulation source to saidoptical means for mode locking said laser beam, ind means for varyingsaid modulation voltage for adjusting the magnitude of the power in thesecond harmonic of the aser beam.

n x t I: t

1. The method of doubling the optical frequency of a laser consisting ofthe steps of: positioning a single nonlinear crystal in the intracavitypath of the laser beam, and suitably modulating said crystal to producemode locking and frequency doubling of the laser beam.
 2. AppaRatus forproducing a laser beam having a frequency that is the second harmonic ofthe fundamental laser frequency comprising, a laser oscillatorcomprising a laser medium and spaced mirrors defining a laser-resonantcavity of predetermined length determinative of the fundamentalfrequency of the laser beam, and a single electro-optic crystal disposedwithin said cavity for simultaneously mode locking said beam and forinteracting with the latter to produce the optical second harmonicthereof, and means for electrically modulating said crystal.
 3. Theapparatus according to claim 2 in which one of said mirrors is totallyreflecting for a laser beam at both the fundamental frequency and thesecond harmonic thereof, the other of said mirrors being totallyreflecting for a laser beam at the fundamental frequency and at leastpartially transmitting at the second harmonic thereof.
 4. The apparatusaccording to claim 2 in which said crystal is a birefringent element,having an ordinary axis and an extraordinary axis, a polarizer withinsaid cavity aligned with the laser beam, said polarizer having apolarization plane aligned with one of said axes of said element wherebythe fundamental mode of the laser beam oscillates in the plane of saidpolarizer and the second harmonic thereof oscillates in the plane of theother of said element axes.
 5. The apparatus according to claim 2wherein the means for electrically modulating comprises a source ofmodulation voltage having a frequency approximately equal to thedifference between the frequencies of adjacent modes in the laser beam,means for coupling said voltage from the modulation source to saidoptical means for mode locking said laser beam, and means for varyingsaid modulation voltage for adjusting the magnitude of the power in thesecond harmonic of the laser beam.